As the chip integration level increases, the application of low power techniques in the design of systems on chips (SoCs) increasingly draws the attentions of researchers. As more and more functional modules are integrated in a single chip, the requirement for chip power consumption becomes higher and higher. Especially, the requirement for steady-state power consumption of wireless sensor network node chips becomes rigorous increasingly: the node chips must be able to operate continuously for months or even years; in most of the time, the node chips are in standby or sleep mode; thus, the requirement for steady-state power consumption is very rigorous.
A Power-On-Reset circuit (POR) is an indispensable part of a SoC, and it provides a global reset signal to the system in the initial stage when the entire chip is powered on, to ensure the entire system can start up from a deterministic state; in addition, when the circuit operates normally, system reset will also be induced to prevent the system from operating in an unstable state if the supply voltage drops to an extremely low level. The POR circuit should always be connected to the power supply, no matter whether the system is in the power-up, power-down, or stable stage. Therefore, an extremely low-power consumption but high-performance POR circuit design is of great importance to the SoC chip.
A simple POR circuit is usually implemented by means of the RC charge/discharge principle. When the supply voltage begins to rise, the reset signal is kept at low level since no sudden change of voltage is permitted at both ends of the capacitor C; as the supply voltage charges the capacitor C until the voltage of the upper plate of the capacitor reaches to the threshold voltage of inverter in the next stage, the output of reset signal will quickly change from low to high and keep at high level; thus, the reset process is finished. Though such a POR circuit is simple in structure, the pull-up voltage is unstable, and the large capacitor is difficult to realize inside chips.